The invention relates to salt mixtures for storing thermal energy in the form of heat of phase-transformation. The invention further relates to the use of said salt mixtures as latent heat storage media for storing and utilizing the waste heat of motor vehicle engines and to corresponding heat reservoirs for use in motor vehicles.
Thermal energy originating from any energy system is utilizable if it can be stored and, when required, can again be retrieved from the reservoir. This type of heat supplier includes, for example, solar radiation energy, the waste heat of any machines, e.g. of motor vehicle engines, process heat, or the electrical energy often supplied at lower cost during the hours of the night, for reasons of energy management and tariffs (off-peak electricity).
Thus, in particular, it is advantageous to equip motor vehicles with a heat reservoir, for example by means of integration into the oil and/or coolant circuit, in that after a prolonged standstill the cooled-down engine can be rapidly brought back up to its operating temperature by the heat stored from previous running. As a result, the warm-up period of the engine is considerably reduced. As is well known, the warm-up period of motor vehicle engines involves increased fuel consumption and less favorable exhaust gas data and increased engine wear.
It is known that, as a substance melts, i.e. during the transition from the solid to the liquid phase, heat is consumed, i.e. absorbed, which, as long as the liquid state persists, is stored in latent form, and that said latent heat is released again during solidification, i.e. during the transition from the liquid to the solid phase. It is then important to find materials whose melting temperature is in a range which corresponds to the temperature level of the supplied heat and which have a high heat of phase transformation. For the purpose of storing the engine waste heat of motor vehicles it is thus desirable, e.g., for the melting temperature of the material to be in the range of 70.degree.-80.degree. C., preferably at approximately 75.degree. C. Furthermore, such materials must be stable to beyond the range of the maximum operating temperature of motor vehicles (approximately 125.degree. C.) and must not have a corrosive effect on the motor vehicle units in question.
It is furthermore a requirement for such latent heat storage materials to remain stable over any number of melting and solidification cycles and not to exhibit any changes in the phase transformation point and the heat of transformation.
DE-A-39 29 900 proposes phase change materials which are based on mixtures of magnesium nitrate with nitrates of the alkali metals or alkaline earth metals. For storing the engine waste heat of motor vehicles in particular, mixtures of magnesium nitrate hexahydrate (Mg(NO.sub.3).sub.2 .multidot.6 H.sub.2 O) and lithium nitrate [sic] (LiNO.sub.3) in a mass ratio of approximately 9:1 are recommended. For a mixture defined as comprising 92-87 g of Mg(NO.sub.3).sub.2 .multidot.6 H.sub.2 O and 8-13 g of LiNO.sub.3 there is quoted there, for example, a melting temperature of 70.degree..+-.2.degree. C. and a heat of fusion of 54.32 Wh/kg (corresponding to 195.6 J/g).
It was found, however, that the known phase change material has a tendency to phase separation even after a few melting and solidification cycles. It is then no longer melted completely, owing to which its heat of transformation and the storage capacity are drastically reduced.
More detailed studies on these mixtures have shown, in addition, that the heat of phase transformation is only about 158 J/g and is thus not optimal. Another unfavorable factor is that the melting temperature is just at the bottom end of the desirable range.